The first two calls you think are just a sort of weird postmodern prank where the lack of meaning is the point. The third call catches you bored, and when you find out you can't trace it back you get somewhat interested. It's obvious that the message was a Social Security number, but you don't understand what that person is supposed to mean.
Two days later he dies.
You don't know even approximately what's going on, but you're not surprised when you get a fourth call.